Computer systems may be subject to security attacks or exploits. For example, a Cross Site Request Forgery (CSRF) attack may involve a malicious website causing a browser of a user to send an unauthorized command to a web server using an existing logged-on session. In this manner, the CSRF attack may access private data of the user, or may initiate changes on the web server without the user's knowledge or approval. For example, a malicious website can trigger the browser to send a request to an email provider to delete a particular email.